Monday, March 17, 2014

Native Species Profile - Common Garter Snake

Today (March 17th) is Saint Patrick's Day.  According to legend, Saint Patrick is responsible for the lack of snakes in Ireland.  He was in the middle of a 40 day fast when he was attacked by snakes.  Saint Patrick arose from his fast and drove all the snakes into the ocean, banishing them from Ireland.  To this day, Ireland has not snakes.

In reality, there were no snakes in Ireland for Patrick to chase into the ocean.  Ireland was covered with glaciers during more than one Ice Age.  During the most recent glacial maximum, three quarters of the island was covered with ice and the remainder was too cold for snakes to survive.  Because Ireland is not connected to the remainder of Europe, no snakes have been able to make the migration to the island in the 11,000 years since the glaciers receded.

Mid-Michigan was also covered with glaciers during this last Ice Age and any snake species had to retreat southward or perish.  Fortunately, Michigan is not an island and the snakes have been able to return in the ensuing 11,000 years.

The most common snake in Mid-Michigan is the Common Garter Snake (Thamnophis sirtalis).

A small Common Garter Snake in a Mid-Michigan woodland

Also known as the Eastern Garter Snake, the Common Garter Snake is found throughout North America from the Atlantic to the Pacific.  A highly adaptable species, the Common Garter Snake is found in most habitat types except dessert, high alpine, and tundra.  In Michigan its inhabits woodlands, wetlands, prairies, farmlands, and suburbs.  Because it is an adaptable species, it is even able to survive in urban areas.

Sometimes, Garter Snakes are mistakenly called "garden snakes".  Garter Snakes are named after  parallel yellow bands that run the entire length of their body.  Besides the Common Garter Snake there are two other species of garter snake found in Michigan.  It is fairly easy to distinguish between the three species.  The Northern Ribbon Snake (T. sauritus) looks more slender and whip-like than the the Common Garter Snake.  Ribbon Snakes also have much longer tails than Common Garter Snakes.  Butler's Garter Snake (T. butleri) is generally smaller, with a thick neck and a small head.

Common Garter Snake in a marsh on North Manitou Island

Common Garter Snakes are highly variable in color.  Color morphs range from brown to green to yellow.  Because of this range of color they can be somewhat confusing to identify at times.  The Common Garter Snake is a medium sized snake, measuring from 16 to 40 inches as an adult. Females are generally larger than males.

Common Garter Snakes are a medium sized snake reaching 16 to 40 inches in length.

One of the reasons that Common Garter Snakes have been so successful is their ability to adapt to a variety of food sources.  Common Garter Snakes are predators.  Smaller individuals prey upon invertebrates such as slugs, worms, and large insects - I have seen one eating a large dragonfly.  Larger individuals eat small vertebrates such as minnows, rodents, birds, and amphibians.  They seem to be especially fond of frogs.  Several times I have found Common Garter Snakes in the process of eating frogs - like the pictures below of one eating a Gray Tree Frog (Hyla versicolor).  Garter Snakes (like all other snakes) swallow their food whole.  While they may use their body to hold down a prey animal they are not constrictors - they do not coil around their prey to suffocate it.  Instead they grasp their prey with their mouths and work their jaws around until they are able to swallow the prey animal head-first.  Garter Snakes do produce a mild venom that might also help them subdue their prey.

Common Garter Snake preying on a Gray Treefrog

Common Garter Snake preying on a Gray Treefrog - swallowed up to the shoulders

Common Garter Snake preying on a Gray Treefrog - swallowed past the shoulders
Common Garter Snakes are solitary species.  Although you may find large numbers of individual snakes in a small area if there is abundant food, each snake acts independently to survive.  There are two times of the year when the snakes may congregate in numbers.  In the fall Common Garter Snakes (and other species of snakes) gather in groups at hibernaculums where the they will hibernate in groups. 

In the Spring, smaller groups are often found together in mating clusters.  If you see a writhing cluster of snakes in the Spring look at it closely.  One of the snakes in the group will be larger - this is a female.  All of the other snakes in the cluster will be smaller males competing for the opportunity to mate.

A mating cluster of Common Garter Snakes

A mating cluster of Common Garter Snakes consisting of one large female and three smaller males.

Common Garter Snakes are generally docile and rarely bite when handled.  I have noticed that small individual snakes tend to be more aggressive than large individuals.  However, as with any wild animal, care should be taken if you choose to handle them.  I mentioned that Garter Snake bites do contain a small amount of mild venom.  Most people, if bitten, will react to a bite with only minor irritation.  However, some people are allergic to this venom and may experience more severe reactions. 

Basic Information

Common Garter Snake 
Thamnophis sirtalis

Size:  16-40” long 

Habitat:  almost all habitats, wetlands, fields, prairies, forests, agricultural lands, suburban areas, etc.

Eats:  amphibians, worms, minnows, slugs, small animals

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